Driving in the UK

I am currently a driver in the US with a valid license... how does this transfer over on a fiancé visa ? Can I drive there or do I need to take lessons and get a provisional license?

You can drive for one year from the date you enter the UK on your fiancé visa.

During that year, you'll need to apply for a provisional license, take the theory (written) test, take some lessons, and take the practical (driving) test.  Do NOT underestimate how long wait times can be.  As you can't work or volunteer while on a fiancé visa, I'd highly recommend trying to knock out as much of the process then as you can. 

I can drive before I apply for a provisional licenss? Also where do I apply for all of this

Yes, if you have your US license you can drive before you have your provisional (you are a licensed driver after all).

Google DVLA.  You apply online.

I can drive before I apply for a provisional licenss? Also where do I apply for all of this

Yes, your US licence is good for up to a year.  This gov.uk site is a good place to look at applying for a provisional licence.

https://www.gov.uk/apply-first-provisional-driving-licence

You can drive for one year from the date you enter the UK on your fiancé visa.

During that year, you'll need to apply for a provisional license, take the theory (written) test, take some lessons, and take the practical (driving) test.  Do NOT underestimate how long wait times can be.  As you can't work or volunteer while on a fiancé visa, I'd highly recommend trying to knock out as much of the process then as you can. 

I second this. I waited for lessons because finances, then life happened. I now have taken a few and my instructor thinks I'm ready due to my prior experience driving (theory passed already). I passed my theory end Of September, I couldn't get a practical test until December. There were no Saturday tests all the way up until April. So, I now have to take time off work. If you have the finances to take lessons (around me they run about 25 pounds per hour) I would recommend it. I also had trouble getting an instructor due to them being booked solid. It took me nearly a month from calling around to actually getting a lesson.